std::tuple_size<std::tuple>

STD：：元组[医]大小<std：：tuple>

Defined in header
template< class T > class tuple_size; /undefined/	(1)	(since C++11)
template< class... Types > class tuple_size< std::tuple<Types...> > : public std::integral_constant<std::size_t, sizeof...(Types)> { };	(2)	(since C++11)
Defined in header <tuple>
Defined in header <array> Defined in header <utility>		(since C++17)(since C++17)
template< class T > class tuple_size<const T> : public std::integral_constant<std::size_t, tuple_size<T>::value> { };	(3)	(since C++11)
template< class T > class tuple_size< volatile T > : public std::integral_constant<std::size_t, tuple_size<T>::value> { };	(4)	(since C++11)
template< class T > class tuple_size< const volatile T > : public std::integral_constant<std::size_t, tuple_size<T>::value> { };	(5)	(since C++11)

以编译时常量表达式的形式提供对元组中元素数的访问。

除了通过包含<tuple>标头时，模板%283-5%29可用。<array>或<utility>包括在内。

所有的专门化std::tuple_size满足感UnaryTypeTrait带着基本特征std::integral_constant<std::size_t, N>对一些人来说N...

The cv-T templates (3-5) are SFINAE-friendly: if std::tuple_size::value is ill-formed when treated as an unevaluated operand, (3-5) do not provide the member value. Access checking is performed as if in a context unrelated to tuple_size and T. Only the validity of the immediate context of the expression is considered.	(since C++17)

辅助变量模板

template< class T > inline constexpr std::size_t tuple_size_v = tuple_size::value;		(since C++17)

继承自STD：积分[医]常量

成员常数

value static	sizeof...(Types) (public static member constant)

成员函数

operator std::size_t	converts the object to std::size_t, returns value (public member function)
operator() (C++14)	returns value (public member function)

成员类型

Type	Definition
value_type	std::size_t
type	std::integral_constant<std::size_t, value>

例

二次

#include <iostream>
#include <tuple>
 
template <class T>
void test(T t)
{
    int a[std::tuple_size<T>::value]; // can be used at compile time
 
    std::cout << std::tuple_size<T>::value << '\n'; // or at run time
}
 
int main()
{
    test(std::make_tuple(1, 2, 3.14)
}

二次

产出：

二次

另见

std::tuple_size	obtains the size of an array (class template specialization)
std::tuple_size<std::pair> (C++11)	obtains the size of a pair (class template specialization)
std::get(std::tuple)	tuple accesses specified element (function template)
tuple_element	obtains the type of the specified element (class template specialization)

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